In power plants of the type comprising a boiler with an integral superheater, a turbine and a separate superheater between the turbine and the integral superheater, it is desirable to drive the superheating of the steam as high as possible to achieve a high thermal efficiency and good heat economy. However, problems have been experienced in prior art plants when the temperature of the steam considerably exceeds 500.degree. C. Under such conditions, ashes from the flue gases passing through the superheaters frequently contain vanadates and other compounds with low melting points which are extremely corrosive in the molten condition. Such molten compounds cause coatings to form on the tubular walls of the superheater and there give rise to high-temperature corrosion. A practical upper temperature limit in the case of oil fired boilers has been found to be around 540.degree. C.